As is known, in weaving processes the yarn can be fed to a textile machine, e.g. a circular knitting machine, from a plurality of yarn feeders which are provided with a storage drum which bears a plurality of yarn loops wound around it which are adapted to be unwound upon request from the downstream machine. As the yarn is unwound from the drum, it can be reloaded either by a motorized arm which rotates like a swivel about an axis that is coaxial to the axis of the drum, or, in the case of the feeders to which reference is made here, by rotating the drum which, therefore, in this case, is motorized.
As is well known to the person skilled in the art, it is extremely important to maintain the reserve of yarn on the drum substantially constant at an optimal value, in order to stabilize the tension of the yarn in output from the yarn feeder. In fact, reducing the reserve below the optimal quantity would cause an excessive increase in the tension of the yarn in output, with the result that defects will be generated in the finished garment. By contrast, an increase in the reserve beyond the optimal quantity can lead to an accumulation of yarn on the end part of the drum, with uneven superimposition of the loops and consequent irregularities in the feeding process.
In EP 2 592 032, the rotation of the motor is controlled so as to maintain the quantity of yarn substantially constant with respect to a preset reserve quantity which is wound on the drum during an initial loading procedure. The yarn feeder is provided with a sensor which is arranged at the output end of the drum and is provided with three or more photocells which are fixed angularly equidistant about the axis of the drum in order to detect the passage of the yarn. On the basis of the sequence with which the photocells are activated, the control unit determines whether the yarn is unwound or wound and controls the motor so that, during normal operation, the quantity of yarn wound is equal to the quantity of yarn unwound, in which case the sensor will not detect the passage of any turn of yarn. Therefore, under operating conditions, an external observer will see the yarn substantially stationary (in the direction of rotation), since the speed for winding in one direction will be the same as the speed of unwinding in the opposite direction.
As a consequence, in the system disclosed in EP 2 592 032, the sensor on its own is not capable of providing absolute information on the quantity of yarn which is unwound from the drum, but only information that corresponds to the quantity of yarn which is wound.